netdev - Re: [PATCH net-next v3 1/4] net: stmmac: Switch to zero-copy in non-XDP RX path

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Message-ID: <20250126205625.00006d64@gmail.com>
Date: Sun, 26 Jan 2025 20:56:25 +0800
From: Furong Xu <0x1207@...il.com>
To: Ido Schimmel <idosch@...sch.org>
Cc: Andrew Lunn <andrew@...n.ch>, Brad Griffis <bgriffis@...dia.com>, Jon
 Hunter <jonathanh@...dia.com>, netdev@...r.kernel.org,
 linux-stm32@...md-mailman.stormreply.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Alexander Lobakin <aleksander.lobakin@...el.com>, Joe Damato
 <jdamato@...tly.com>, Andrew Lunn <andrew+netdev@...n.ch>, "David S.
 Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub
 Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Maxime
 Coquelin <mcoquelin.stm32@...il.com>, xfr@...look.com,
 "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH net-next v3 1/4] net: stmmac: Switch to zero-copy in
 non-XDP RX path

On Sun, 26 Jan 2025 13:35:45 +0200, Ido Schimmel wrote:

> On Sun, Jan 26, 2025 at 06:37:14PM +0800, Furong Xu wrote:
> > The "SPH feature" splits header into buf->page (non-zero offset) and
> > splits payload into buf->sec_page (zero offset).
> > 
> > For buf->page, pp_params.max_len should be the size of L3/L4 header,
> > and with a offset of NET_SKB_PAD.
> > 
> > For buf->sec_page, pp_params.max_len should be dma_conf->dma_buf_sz,
> > and with a offset of 0.
> > 
> > This is always true:
> > sizeof(L3/L4 header) + NET_SKB_PAD < dma_conf->dma_buf_sz + 0  
> 
> Thanks, understood, but are there situations where the device is
> unable to split a packet? For example, a large L2 packet. I am trying
> to understand if there are situations where the device will write
> more than "dma_conf->dma_buf_sz - NET_SKB_PAD" to the head buffer.

Nice catch!
When receiving a large L2/non-IP packet, more than "dma_conf->dma_buf_sz
- NET_SKB_PAD" will be written.

So we should:
pp_params.max_len = dma_conf->dma_buf_sz + stmmac_rx_offset(priv);

Thanks a lot, Ido
Have a nice weekend :)